3D Rayleigh Index Fields in Swirl Flames Measured from Tomographic Reconstruction of Doubly-Phase-Conditioned OH* Chemiluminescence

A method of calculating three-dimensional Rayleigh Index (RI) fields has been developed for swirl flames that undergo combustion dynamics featuring 3D rotating asymmetries, namely helical precessing vortex cores using... [ view full abstract ]

A method of calculating three-dimensional Rayleigh Index (RI) fields has been developed for swirl flames that undergo combustion dynamics featuring 3D rotating asymmetries, namely helical precessing vortex cores using synchronised single-view chemiluminescence measurements and pressure oscillation measurements. Study of RI fields is critical to the understanding of thermoacoustic oscillations in lean premixed gas turbine combustors. However, determining these fields experimentally is difficult due to the need for high temporal resolution 3D heat release rate measurements. In this work, RI fields are calculated using one high-speed intensified camera to measure instantaneous OH* CL fields and one acoustic microphone for measuring pressure fluctuations. 3D RI fields are calculated by performing a tomographic reconstruction of doubly-phase resolved mean OH* chemiluminescence fields. OH* fields were phase-resolved based on the periodic oscillation OH* centroid location and the periodic oscillations in pressure. Tomographic reconstructions are performed using algebraic reconstruction techniques (ART) as well as tailored techniques developed by Moeck et al (Exp. Fluids 54:1498) were used to produce a 3D OH* emission field at each phase in the acoustic cycle, taken to be an analogue of heat release rate. Total RI fields are calculated by integrating the product of total heat release rate and acoustic pressure over one thermoacoustic cycle. Accuracy of the different reconstruction techniques used was compared by performing reconstructions of simulated flame phantoms, as well as comparing reconstructions produced from experimental measurements of a lean premixed swirl flame with high-energy thermoacoustic oscillations. The expected trends in both total RI and structures of the RI fields reveal the expected results, featuring considerable coupling in both the inner and outer recirculation zones caused by the helical shape of the experimental flames studied.